1. Field of the Invention
The present invention relates to a prism assembly and a method for producing the prism assembly. More specifically, the present invention relates a prism assembly which performs color separation and color synthesis in, for example, an optical system in a liquid crystal projector or the like.
2. Description of the Related Art
Conventionally, in an optical system in a liquid crystal projector or the like, a prism assembly which performs color separation and color synthesis is used.
As shown in FIGS. 23A and 23B, a conventional prism assembly 90 of this type includes, for example, prisms 91 to 94 made of glass, and dichroic layers 96 and 97 formed on specific planes of the prisms 91 to 94.
In the conventional prism assembly 90, as shown in FIGS. 23A and 23B, red light is introduced through a light incidence plane denoted by the reference symbol "R light incidence plane"; green light is introduced through a light incidence plane denoted by the reference symbol "G light incidence plane"; and blue light is introduced through a light incidence plane denoted by the reference symbol "B light incidence plane". Also, red, green and blue lights are emitted through a light emission plane denoted by the reference symbol "RGB lights emission plane". Incidentally, each of these reference symbols denotes the same plane in the following description.
The conventional prism assembly 90 having the above-mentioned constitution is produced by the following method.
Incidentally, in the following description, each of the prisms constituting the prism assembly has five planes: three rectangular side planes and two right-angled-triangle-shaped base planes. Of these side planes, two side planes including the sides forming the right angle of the right-angled-triangle-shaped base plane are referred to as a "first plane" and a "second plane", while the side plane including the hypotenuse of the triangle serving as the base plane is referred to a "third plane".
Firstly, a glass block is roughly processed by pressing or cutting so as to form a rough contour of a prism of the prism assembly 90.
Next, the roughly processed glass block is attached on a plate by means of paraffin, and then the first plane of the prism is processed.
The processing of the first plane includes rough shaving to a predetermined thickness, semi-finishing, and finishing, performed in this order. In the rough shaving, abrasive grains of C# 80 are used; in the semi-finishing, abrasive grains of A# 120 to 280 are used; and in the finishing, abrasive grains of A# 400 to 600 are used.
Thereafter, the paraffin is removed from the prism, and the prism is washed.
After the first plane is processed, the second plane and then the third plane are processed in the same steps as in the first plane.
In this way, the prisms 91 to 94 are produced.
Incidentally, a stock allowance for sanding and polishing each of the side planes is expected to be 0.07 to 0.1 mm for each of the side planes.
Thereafter, dichroic films 96 and 97 are directly coated on specific planes of the glass prism 92. Then, the glass prisms 91 to 94 are joined to form the prism assembly 90.
However, in the glass prisms 91 to 95 thus processed, the opposed sides of each plane are unfavorably liable to fail to be in parallel, so that pyramidal errors (falling-down errors) are liable to occur.
To avoid such errors, the producer must process the prisms 91 to 94 while paying attention to the following points in each of the steps of rough shaving, semi-finishing, and finishing. That is, in the processing of the third plane performed after processing the first and second planes, the producer must pay attention to causing the two angles formed between the third plane and the first and second planes (i.e., the two angles formed at both ends of the hypotenuse of the right-angled triangle-shaped base plane) to become equal to each other, and to causing the angles formed between the first to third planes and the upper and lower base planes to become right angles.
Because of the constitution and the production method as described above, in the conventional prism assembly and the conventional method for producing the prism assembly, a skilled expert needs to take a long time in order to produce a prism assembly having satisfactory precision in prism angles and planes. Consequently, according to the conventional method, it is difficult to produce such a prism assembly efficiently.